Method of inhibiting the deposition of scale in an aqueous system

ABSTRACT

A composition for inhibiting deposition of scale in aqueous systems is disclosed which comprises carboxymethylcellulose and gluconic acid or glucoheptonic acid or both, or their water soluble salts. The composition is incorporated in the aqueous system in concentrations sufficient to substantially completely prevent scale deposition.

This is a division of application Ser. No. 709,484, filed Mar. 7, 1985,now U.S. Pat. No. 4,609,483.

BACKGROUND OF THE INVENTION

This invention relates to an improved composition for inhibitingdeposition of scale and sludge in aqueous systems. More specifically,small quantities of a composition consisting of sodiumcarboxymethylcellulose ("CMC") and either gluconic acid or glucoheptonicacid or both, or their water soluble salts are added to the steamgenerating systems to prevent the formation of scale and sludge depositstherein.

The formation of scale and sludge deposits on heating surfaces is themost serious problem encountered in boiler operations and is the resultof entrained scale-forming impurities, such as calcium and magnesiumsalts, and iron. External softening of boiler feedwater partiallyremoves these substances, but internal chemical treatment is typicallyapplied in industrial boilers and other aqueous systems in order tocounteract scale formation. The chemical treatment of boiler watergenerally involves the combined use of a precipitating agent and asludge conditioner to maintain the boiler sludge in a fluid form. Thechemicals most commonly employed for precipitation of calcium salts aresoda ash and the sodium phosphates, for example disodium phosphate,trisodium phosphate and sodium metaphosphate. Magnesium salts areprecipitated as magnesium hydroxide by the alkalinity of the boilerwater.

The precipitate, or sludge, is rendered non-sticky and easily manageableby the addition of a sludge conditioner, and it is removed from theboiler periodically through the blowdown. Compounds such aspolyacrylates, polymethacrylates and carboxymethylcellulose have beenused as sludge conditioners in certain settings, such as in foodprocessing plants and in U.S. Dept. of Agriculture (USDA) inspected meatand poultry plants. However, in many food processing plants it isrequired to use only those boiler water additives which are generallyrecognized as safe ("GRAS") by the Food and Drug Administration ("FDA").For example, GRAS substances are often preferred where the steamproduced in the boiler or steam generating system will contact the foodbeing processed. Polyacrylates and polymethacrylates are not GRASsubstances.

Sodium-CMC is a GRAS substance and previously has been used for sludgeconditioning, either alone or in combination with other substances. Forexample, U.S. Pat. No. 3,188,289 (Kahler et al.) discloses the reductionor prevention of deposits in steam generating systems by introducing thesodium salt of CMC into the feedwater. U.S. Pat. No. 3,483,033 (Casey)discloses minimizing the formation of scale deposits during evaporationof raw cane and beet sugar juices by adding a composition comprising ahydrolyzed polyacrylamide, a protective colloid such as sodium alginateor CMC and preferably one or more other ingredients selected from thefollowing: EDTA, a gluconate such as for example sodium-gluconate orsodium gluco-heptonate, and polyphosphates. U.S. Pat. No. 4,085,060(Vassileff) discloses a composition capable of sequestering metal ionswhich is a mixture of at least two different hydrophilic polymers, oneof which may be CMC with a molecular weight between 600 and 60,000.

Gluconic and/or glucoheptonic acid have been used as sequestering orchelating agents. U.S. Pat. No. 3,308,062 (Gunther) discloses the use ofgluconic acid in steam boilers, evaporators, stills and the like in anamount to prevent the formation of scale. U.S. Pat. No. 3,328,304(Globus) discloses sequestering and/or chelating agents useful in thedairy, brewery and pharmaceutical industries which are prepared byadmixture of citric acid (anhydrous), D-gluconic acid (primarily as thelactone), magnesium hydrocarbonate, magnesium acid citrate, and waterboth in the combined and free states.

Experience has shown that sodium-CMC alone is not completely effectivein preventing the formation of boiler scale. Indeed, the compositionsdiscussed above comprise both CMC and one or more other compounds, notall of which are GRAS substances. The composition described herein is animproved sludge conditioner in which the sludge conditioningcapabilities of CMC alone have been improved by the inclusion ofgluconic acid or glucoheptonic acid or their water soluble salts.

SUMMARY OF THE INVENTION

The improved sludge conditioning composition of this invention comprisescarboxylmethylcellulose and either gluconic acid or glucoheptonic acidor both, or their water soluble salts. The composition is capable ofpreventing and controlling scale deposition in aqueous systems, such assteam generating systems, even when used in small quantities. Eachcomponent of the conditioning composition is a GRAS substance and may beused in the preparation of steam generated for contacting food in foodprocessing operations.

It is a primary purpose of this invention to provide a composition whichefficiently and effectively prevents the formation and deposition ofscale and sludge deposits in aqueous systems, most particularly in steamgenerating boilers.

It is an additional purpose to provide a composition of thesecharacteristics which consists of GRAS substances and which is suitablefor use in food processing operations where the generated steam willcontact the food or food ingredients.

Another purpose is to provide a composition which has sufficientlyenhanced efficiency that it prevents scale deposition even when used insmall quantities.

DESCRIPTION OF THE INVENTION

The scale inhibiting composition disclosed and described herein isintended for use in aqueous systems in which scale deposition isproblematic. The composition is ideally suited for use in a steamgenerating system, particularly where the steam is to be used in foodpreparation, for the purpose of inhibiting the formation and depositionof scale on heating surfaces. The composition consists of CMC and eithergluconic acid or glucoheptonic acid or both, or their water solublesalts. The composition may be used as an additive to the boilerfeedwater or may be added directly to the boiler itself.

CMC, one of the components of the composition of this invention, is awater soluble polymer. The polymer is used as a water soluble salt ofthe carboxylic acid, preferably the sodium salt. The CMC utilized mustbe of sufficient purity for the intended use of the final composition.That is, if the intended use is in a steam generation system for foodprocessing, U.S.P. (United States Pharmacopeia) grade CMC or food gradeCMC must be used, etc. CMC having a molecular weight of about 90,000 toabout 500,000 should be selected for the present composition. It ispreferred that the molecular weight be in the range of about 200,000 toabout 300,000.

The second component is either gluconic acid or glucoheptonic acid, or acombination of gluconic acid and glucoheptonic acid. Any of the watersoluble salts of these acids may be utilized, provided that they arecompatable with the intended end use, but the sodium salts arepreferred. As with the CMC component, the required purity of thegluconic and/or glucoheptonic acid component will depend on the settingin which the composition is used.

It is preferred to use approximately equal portions of the twocomponents, that is, about 50% CMC and about 50% of the gluconic acidand/or glucoheptonic acid component. However, if desired or if necessarybased on considerations of cost or availability, it will be acceptableto use from about 1 to 10 parts of CMC to about 10 to 1 parts of thegluconic acid and/or glucoheptonic acid component.

The components may be added to the feedwater or boiler water as apre-mixed composition. The pre-mixed composition is quite viscous andpreferably is mixed in water to achieve a free-flowing aqueous solution.If desired, the two component scale inhibiting composition may be mixedwith other boiler water treatment chemicals, such as phosphate, sulfiteand caustic soda, for example. Alternatively, the two components may beadded separately to the aqueous system to form the scale-inhibitingcomposition in situ.

The composition described above is intended for use as an additive inaqueous systems, such as boiler water in steam generating facilities, toinhibit and prevent the formation of scale and sludge deposits. Sincethese deposits are most troublesome on and near heating surfaces, thescale-inhibiting composition of this invention should be present in theboiler water at a point at or prior to the water's contact with heatingsurfaces. The composition may be added to the boiler feedwater or may beadded directly to the boiler, as convenient. The scale inhibitingcomposition should be present in the boiler water in concentrations ofat least about 0.1 ppm, preferably about 5.0 to about 50.0 ppm, foreffective scale and sludge control.

The use of the scale inhibiting composition in this manner effectivelyconditions the boiler water precipitate to inhibit and prevent formationof scale and sludge deposits. The precipitate or sludge is renderednon-sticky and loses its tendency to adhere to the surfaces present inthe aqueous system. Rather, the conditioned sludge is easily manageableand may be removed from the system periodically, as necessary andconvenient.

The example which follows is given for illustrative purposes and is notmeant to limit the invention described herein. The followingabbreviations have been used throughout in describing the invention:

CMC--carboxymethylcellulose

FDA--Food and Drug Administration

ft² --square feet

gm--gram(s)

GRAS--generally recognized as safe

hr--hour

%--percent

ppm--part(s) per million

psig--pounds per square inch gauge

USDA--United States Department of Agriculture

USP--United States Pharmacopeia

EXAMPLE

This experiment compared the boiler water scale inhibiting properties ofthe composition of this invention, with sodium-CMC alone, sodiumgluconate alone, sodium glucoheptonate alone, and no additive at all.The boiler feedwater was prepared by diluting Lake Zurich, Ill. tapwater with distilled water to 40.0 ppm total hardness as calciumcarbonate, followed by adjustment to a 6:1 elemental calcium tomagnesium ratio using calcium chloride. Three volumes of feedwater toone volume of a chemical treatment solution were fed to the boiler foreach test, giving a feedwater total hardness of 30.0 ppm as calciumcarbonate. The composition of the chemical treatment solution wasadjusted prior to addition to the feedwater to give concentrations inthe boiler water as indicated in Table I. Boiler blowdown was adjustedto 10% of the boiler feedwater, giving approximately ten-foldconcentrations of the boiler water salines as compared to the feedwaterconcentrations. The boiler water also comprised 30.0 ppm of one of thesludge conditioning additives listed in Table II.

                  TABLE I                                                         ______________________________________                                        (Boiler Water Composition)                                                    Ingredient          Quantity*                                                 ______________________________________                                        Sodium Hydroxide as NaOH                                                                         258         ppm                                            Sodium Carbonate as Na.sub.2 CO.sub.3                                                            120         ppm                                            Sodium Chloride as NaCl                                                                          681         ppm                                            Sodium Sulfite as Na.sub.2 SO.sub.3                                                              50          ppm                                            Sodium Sulfate as Na.sub.2 SO.sub.4                                                              819         ppm                                            Silica as SiO.sub.2                                                                              <1          ppm                                            Iron as Fe         100         ppm                                            Phosphate as PO.sub.4                                                                            10-20       ppm                                            ______________________________________                                         *Measured as parts per million in the boiler water (after the tenfold         concentration).                                                          

                  TABLE II                                                        ______________________________________                                        (Scale Reduction Evaluation)                                                                    Dosage  Scaling                                                               in      Rate                                                Sludge Conditioning                                                                             Boiler  gm/(ft.sup.2)                                                                           Scale                                     Additive          Water   (hr)      Red'n                                     ______________________________________                                        Blank (no additive)                                                                             --      0.348     --                                        Sodium glucoheptonate (I)                                                                       30 ppm  0.278     20.1%                                     Sodium gluconate (II)                                                                           30 ppm  0.161     53.7%                                     Sodium CMC (MW 50,000) (III)                                                                    30 ppm  0.215     38.2%                                     Sodium CMC (MW 70,000) (IV)                                                                     30 ppm  0.191     45.1%                                     Sodium CMC (MW 250,000) (V)                                                                     30 ppm  0.087     75.0%                                     I & III (1:2 mixture)                                                                           30 ppm  0.144     58.6%                                     I & IV (1:1 mixture)                                                                            30 ppm  0.142     59.2%                                     I & V (1:4 mixture)                                                                             30 ppm  0.014     96.0%                                     I & V (1:1 mixture)                                                                             30 ppm  0.001     99.7%                                     II & V (1:1 mixture)                                                                            30 ppm  0.000     100.0%                                    ______________________________________                                    

All scaling tests were conducted in the following manner: The testingwas conducted in a small laboratory boiler having three removableelectric heating tubes. The type of boiler used has been describedpreviously in the proceedings of the Fifteenth Annual Water Conference,Engineers Society of Western Pennsylvania, pp. 87-107 (1954). Each testwas run for 47 hours at a boiler pressure of 400.0 psig. At thecompletion of each test, the heating tubes were removed individuallyfrom the boiler. The scale or deposit present on six inches of thecentral length of each tube was removed by scraping, collected in atared vial and weighed. The results, shown in Table II, indicate thatthe tested compositions of this invention--that is, compositionscomprising sodium-CMC and either sodium gluconate or sodiumglucoheptonate--are far superior for reducing scale deposits than theindividual components and the boiler water without any sludgeconditioner. The composition comprising 250,000 MW sodium-CMCsignificantly outperformed the compositions comprising lower molecularweight CMC.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the invention.

We claim:
 1. A method of inhibiting the deposition of scale in anaqueous system comprising the step of adding to or causing to be presentin the system water scale inhibiting amounts of a composition having afirst component selected from carboxymethylcellulose and its watersoluble salts within a molecular weight range of about 90,000 to about500,000, and a second component consisting of at least one compoundselected from gluconic acid, glucoheptonic acid and their water solublesalts; the ratio of the first component to the second component in saidcomposition being between about 1:10 and about 10:1, such that the totalresulting concentration of said first and second components in thesystem water achieves a reduction in scale deposit that is superior tothe reduction in scale deposit achieved by an equivalent concentrationof either component without the other.
 2. The method of claim 1 in whichthe first and second components are sodium salts.
 3. The method of claim1 in which the first and second components of said composition are addedseparately to said aqueous system to form said composition in situ. 4.The method of claim 1 in which said composition is caused to be presentin said aqueous system in concentrations of at least about 0.1 parts permillion.
 5. The method of claim 4 in which said composition is caused tobe present in concentrations of about 5.0 to about 50.0 parts permillion.
 6. The method of claim 1 in which the molecular weight of saidcarboxymethylcellulose is about 200,000 to about 300,000.
 7. The methodof claim 1 in which the salts are those which are recognized as safe foruse at food processing plants.
 8. The method of claim 1 in which saidratio is about 1:1.
 9. The method of claim 1 in which said aqueoussystem is a steam generating system.
 10. The method of claim 9 in whichsaid composition is caused to be present in the boiler water of saidsteam generating system in concentrations of at least about 0.1 partsper million.
 11. The method of claim 10 in which said composition iscaused to be present in concentrations of about 5.0 to about 50.0 partsper million.
 12. The method of claim 9 in which the steam generated isused for processing food or food additives.
 13. The method of claim 9wherein the steam generating system is a pressurized boiler system. 14.The method of claim 13 wherein the boiler is pressurized up to at leastabout 400 psig.
 15. The method of claim 1 wherein the aqueous system isa steam generating boiler system; wherein the first component and thesecond component are added to the system water as a compositionconsisting of said first and second components; and wherein saidcomposition is added to the system water at a concentration of at least0.1 ppm.
 16. The method of claim 15 wherein the steam generating boilersystem operates at about 400 psig.
 17. The method of claim 15 whereinphosphate is also added to the system.
 18. The method of claim 15wherein sulfite is also added to the system.
 19. The method of claim 15wherein caustic soda is also added to the system.
 20. The method ofclaim 15 wherein the composition is added at a concentration of betweenabout 5 and about 50 parts per million.
 21. The method of claim 20wherein the second component is gluconic acid or a water soluble saltthereof.
 22. The method of claim 21 wherein the first and secondcomponents are sodium salts.
 23. The method of claim 20 wherein thesecond component is glucoheptonic acid or a water soluble salt thereof.24. The method of claim 23 wherein the first and second components aresodium salts.
 25. The method of claim 1 wherein at least one watertreatment chemical selected from phosphates, sulfites, and caustic sodais present in the system water.